The formula for Capacitance is:
C = Q / V (where Q = charge, and V = voltage)
Rearrange, the equation so you are solving for V.
V = Q / C
Now plug in your values, Q = 96 µC and C = 4 µF (*since they are both in µ there is no need to convert to SI units, since the units will cancel out, and result in just Volts)
V = 96 µC / 4 µF = 24 V
Similarly for Part B just, replace Q with 36 µC and plug in the equation once more,
V = 36 µC / 4 µF = 9 V